1. Field of the Invention
The present invention relates to the measurement and control of combustion processes and, in particular, systems for determining the ratio between the oxygen-carrying gas content and the fuel content of a mixture for combustion by a primary combustion system by means of a secondary combustion system wherein part of said fuel and part of said oxygen-carrying gas are reacted.
By the term "fuel" as used herein, we mean all kinds of fluid fuels, such as gaseous, vaporous or liquid fuels or mixtures thereof.
2. Prior Art
It is standard practice for combustion conditions and the ratio between the oxygen-carrying gas content and the fuel content of a combustible mixture to be determined by an analysis of the composition of the flue gas in the flue system or by the analysis of a flue gas sample taken from the flue system or by measuring the ratio of the oxygen-carrying gas and the fuel flowing to the primary combustion system, the signal thereby obtained may be displayed for adjusting a burner or burners at certain intervals in response to the values displayed. It is also standard practice for controlled stationary systems to be fitted with a transducer for the conversion of said signal into an appropriate signal for input into a controller controlling the ratio in which said oxygen-carrying gas and said fuel are blended by means of appropriate actuators. If the signal obtained by measurement is generated by a probe in the flue system, said signal lags behind any change in the ratio between the oxygen-carrying gas content and the fuel content of the mixture combusted brought about by an adjustment of actuating elements integrated in the lines taking said oxygen-carrying gas and said fuel to the combustion system, said lag depending on the locations of said actuating elements and said probes in said flue system, the size of the space in which said mixture is combusted, oxygen-carrying gas, fuel and flue gas flows, the combustion process itself and other factors. The response of any system used for controlling the ratio between the oxygen-carrying gas content and the fuel content of the mixture to be combusted thereby depends directly on the sum of delays in the control loop being the sum of delay times and time constants of the various system elements referred to. If such delays are long, control system response will accordingly be slow. Such slow control system response may, in particular in the case of fuel-rich combustion tending to cause sooting and in the case of near-stoichiometric combustion be the cause of high energy loss, high pollutant emission or even damage to the combustion system or the charge of an industrial furnace, if disturbances resulting in a change in combustion conditions occur.
Further, the ingress of undesired secondary air blending with the products of combustion may mainly in the case of room-sealed combustion systems falsify the composition of the flue gas measured in the flue system in conventional combustion-controlled application. If the combustion system burns a fuel-lean mixture, the signals obtained from a system measuring flue gas composition may be falsified by uncombusted flue gas constituents. In the case of unflued combustion systems where the products of combustion blend rapidly with tertiary air to be heated, it is practically impossible to determine the primary oxygen share in the mixture combusted by analyzing the flue gas and it is therefore usual in such cases to measure and/or control the flows of oxygen-carrying gas and fuel to the primary combustion system, the effectiveness of such systems depending largely on the fuel used and thereby the knowledge of the properties of said fuel. Further, simple conventional control systems disregard pressure and temperature variations which have an influence on fuel/air blending and on combustion conditions. Although the determination of characteristic uncombusted constituents of the mixture burnt is theoretically a method of determining the ratio between the oxygen-carrying gas content and the fuel content of a mixture which is valid in the case of pre-mixed combustion systems, said method is again associated with the disadvantage of being highly sensitive to variations in the fuel characteristics.
A method of a type similar to that proposed by the present invention and a corresponding system for nozzle-mixing combustion systems are known from U.S. Pat. No. 4118172. Said patent proposes a secondary combustion system connected in parallel to the main combustion system wherein part of the oxygen-carrying gas and part of the fuel are combusted. In the case of the system which is the subject of the above U.S. patent, the fuel/air ratio of the mixture combusted by said secondary combustion system is varied at regular intervals and the maximum flame temperature is measured to determine the fuel/air ratio required for stoichiometric combustion at constant heat input. The fuel/air ratio for the main combustion system being controlled as a function of the fuel/air ratio for stoichiometric combustion in said secondary combustion system. Said secondary combustion system thereby serves as a pilot combustion system for the main combustion system and the process of combustion in said secondary system is disturbed at regular intervals for determining the fuel/air ratio for stoichometric combustion at constant heat input in said secondary combustion system and for thereby controlling the fuel/air ratio in the mixture for combustion by the main combustion system. The response behaviour of said known system is relatively slow, since prior to a change in the fuel/air ratio for the main combustion system by the control system, several disturbance cycles and control system response cycles in said pilot system will normally pass and heat input must be kept constant. Further, said pilot system is comparatively complex although it is substantially only fit for detecting changes in fuel properties. Finally, said known system does not provide for a correlation between combustion conditions in the main combustion system and in the secondary combustion system.